1. Field of the Invention
The present invention relates to a transmitting circuit apparatus and method used in radio communications.
2. Related Art of the Invention
In a transmitting circuit apparatus used in a digital radio communication using modulation techniques such as QPSK, it is common to use an orthogonal modulator as a modulator. The basic configuration of a conventional transmitting circuit apparatus is shown in FIG. 14. That is, FIG. 14 shows an orthogonal modulator 403, a band pass filter 404, an IQ signal generator 405, a local oscillator 406, a phase shifter 407, mixers 408 and 409, a synthesizer 410, and a power amplifier 411.
The IQ signal generator 405 inputs digital data to divide it into two lines, and generates a baseband I signal and a baseband Q signal, which are analog signals, from respective lines to output the signals to an orthogonal modulator 403 respectively.
The orthogonal modulator 403 comprises the phase shifter 407, mixers 408 and 409, and synthesizer 410.
The local oscillator 406 outputs a sinusoidal wave signal at a carrier frequency, and a limiting signal at the carrier frequency that is outputted is divided into two signals, whose phases are different by 90 degrees from each other, by the phase shifter 407, the two signals which are inputted into the mixer 408 and mixer 409 respectively.
The mixers 408 and 409 perform the amplitude modulation of the signals at the carrier frequency, whose phases differ by 90 degrees from each other, with the baseband I signal and Q signal respectively, the signals that are synthesized by the synthesizer 410 and become an output of the orthogonal modulator 403.
The output of the orthogonal modulator 403 is amplified by the power amplifier 411, and a residual component after reduction of its unnecessary frequency component by the band pass filter is outputted.
In addition, an example of a transmitting circuit apparatus used for an optical base station used in mobile communications etc. is shown in FIG. 15 as another conventional example.
In order to enable a radio terminal to be used in an underground shopping center which an electric wave of a master station does not reach, the optical base station has the configuration of connecting a master station, which has all the control functions of the base station, to a slave station, which is used as a front end to a radio signal, via an optical fiber. Since FIG. 15 shows the similar configuration to that in FIG. 14 except the connection between the orthogonal modulator 403 and power amplifier 411 via an optical fiber, the same reference numerals are given to the same parts and detailed explanation will be omitted.
FIG. 15 shows a master station 421, a slave station 422, an E/O converter 423, an O/E converter 424, and an antenna 420.
In the master station 421, the E/O converter 423 comprising a laser diode converts an output of the orthogonal modulator 421 into an optical signal from an electric signal and the optical signal is transmitted to the slave station 422 through the optical fiber 425.
The slave station 422 converts into an electric signal the optical signal received by the O/E converter 424 comprising a photo diode, amplifies the electric signal by the power amplifier 411, removes an unnecessary frequency component by the band pass filter 404, and transmits the electric signal from an antenna 420.
In this conventional transmitting circuit apparatus, since inputs of the orthogonal modulator 403 are analog signals, it is necessary for the analog signals not to be distorted in the mixers 408 and 409. Therefore, it is difficult to sufficiently enlarge an output level of the orthogonal modulator 403.
In addition, although it is necessary to amplify an output of the orthogonal modulator 403 by the power amplifier 411 since the output level of the orthogonal modulator 403 cannot be sufficiently enlarged, it is necessary to operate the power amplifier 411 in a sufficiently small level to a saturation level since it is necessary to also operate the power amplifier 411 in a linear area with little distortion. Therefore, since the power consumption of the power amplifier 411 is large, it is not possible to make the power consumption of the whole transmitting circuit apparatus small.
In addition, the configuration in FIG. 15 showing an transmitting circuit apparatus of an optical base station which is another conventional example also requires the linearity of the E/O converter 423, optical fiber 425, and O/E converter 422 in addition to the large power consumption of the power amplifier 411. Therefore, although the configuration of the slave station is simple, it becomes difficult to obtain linearity as power consumption increases substantially.
Thus, a conventional transmitting circuit apparatus has an issue that power consumption cannot be made small.